With the growth of the Internet and the like, a large-capacity optical communication system has been advanced. For example, in backbone network systems connecting between communication stations, the research and development have been performed for optical transmitters and optical receivers capable of transmitting a signal exceeding 40 Gbit/s per wavelength. If the bit rate per wavelength increases, the increase in signal quality deterioration is caused by the waveform distortion due to decreased resistance to an optical signal-to-noise ratio (OSNR), chromatic dispersion, polarization mode dispersion, non-linear effect in transmission lines. For this reason, in recent years, a digital coherent receiving method with high resistance to OSNR and waveform distortion has been paid attention to. In the digital coherent receiving method, optical intensity information and phase information are extracted from received signals, and the demodulation is performed by means of a digital signal processing technique. By using the digital coherent receiving method, the OSNR resistance is improved by the coherent receiving method, and the compensation for waveform distortion is realized by the digital signal processing technique. Therefore, it is possible to obtain high reliability even in the optical communication system for transmitting signals exceeding 40 Gbit/s.
Patent Literature 1 describes an example of a coherent optical receiving device used for the foregoing digital coherent receiving method. FIG. 7 illustrates a configuration of a related coherent optical receiving device described in Patent Literature 1. The related coherent optical receiving device 5000 receives an optical received signal 5001 and local oscillation light 5002 having almost the same wavelength as that of the optical received signal 5001 from a local oscillation light source. The local oscillation light 5002 and the optical received signal 5001 are made interfere, converting it into an electrical signal (coherent detection). Since the coherent detection method has strong polarization dependence, a single optical receiver can receive only an optical signal having the same polarization state as that of the local oscillation light. Therefore, a polarization separation unit 5010 is provided in an input part of the optical received signal 5001 and separates the optical received signal 5001 into two polarization components orthogonal to each other. As a result, two optical receivers are required in order to receive a string of optical signal, but such disadvantage can be compensated by doubling the information transmission quantity by means of polarization multiplexing.
Each polarization light of the optical received signal 5001 and the local oscillation light 5002 are inputted into an optical 90-degree hybrid circuit 5100. It is possible to obtain, from the optical 90-degree hybrid circuit 5100, four types of output light in total, that is to say, a pair of output light obtained by making each polarization light component and local oscillation light interfere in phase and reverse phase, and a pair of output light obtained by making them interfere with the phase relationship of orthogonal (90 degrees) and inverse orthogonal (−90 degrees). These output optical signals are converted into current signals by two photodiodes 5200 for each pair and inputted into differential transimpedance amplifiers 5300. As a result, a direct-current component is cancelled and only a beat component generated by the optical received signal 5001 and the local oscillation light 5002 can be efficiently extracted. The electrical signals output from the differential transimpedance amplifiers 5300 are in-phase interference components (I component) and quadrature interference components (Q component) of the optical received signal and the local oscillation light, respectively.
The outputs of each polarization, that is, four types of electrical signals in total composed of I component and Q component of X polarization, and I component and Q component of Y polarization, are analog-to-digital (AD) converted at high speed by analog-to-digital conversion units (ADC) 5400, respectively. After being converted into digital information signals, the digital information signals are inputted into a digital signal processing unit (DSP) 5500. The digital signals obtained in this way can be processed for various equalization/decision processing by the digital signal processing technique which is widely used in wireless communication. After such digital signal processing and error correction processing are performed, information signals with ultrahigh-speed (100 Gbit/s, for example) are output.